You should consider reporting this to both the California Labor Commissioner (Dept of Industrial Relations) and the IRS, both would be extremely interested in this especially if this is DoorDash's standard procedure.

I've used the service a few times in the past and probably will again, but I'm going to start asking the drivers how much they got as a tip and will definitely report any discrepancies.

The website specifically says 100% goes to the delivery person ("Dasher Tip (100% for your Dasher)") so I don't think they can weasel out of that by saying the tips are pooled or any of that other nonsense.